Centrifugal compressors are commonly used for fluid compression in rotating machines such as, for example, a gas turbine engine. Gas turbine engines typically include at least a compressor section, a combustor section, and a turbine section. In general, during operation, air is pressurized in the compressor section then mixed with fuel and burned in the combustor section to generate hot combustion gases. The hot combustion gases flow through the turbine section, which extracts energy from the hot combustion gases to power the compressor section, other gas turbine engine loads, and to provide excess energy for either shaft power or thrust.
A centrifugal compressor is a device in which a rotating impeller delivers air at relatively high velocity through centrifugal force on the gas within the impeller. Such a compressor also includes a diffuser, which normally is an annular space surrounding the periphery of the impeller and which usually is provided with vanes to guide the gas flow in order to recover static pressure, and minimize turbulence and frictional losses in the diffuser. The air or other gas (which will be referred to hereafter as air) is delivered from the impeller with a substantial radial component of velocity and ordinarily a substantially greater tangential component. The function of the diffuser is to decelerate the air smoothly and to recover as static pressure (head) the total or stagnation pressure (dynamic head) of the air due to its velocity.
While centrifugal compressors operate over a variety of flow conditions and ranges, they are designed to operate most efficiently at one set of operating conditions, usually referred to as the design point. For example, a centrifugal compressor may be designed for maximum efficiency and minimum adequate surge margin when operating to supply maximum shaft horsepower. As a consequence of selecting these design conditions, when the compressor is operating off the design point, it operates at reduced efficiency and potentially reduced stall margin. It is therefore desirable to improve the compressor's efficiency and low flow stall margin when operating off the design point. One option for improving efficiency and/or stall margin can be to vary the diffuser area as the operating point of the compressor changes.